After growing fears that Westerners inside Syria were training with al Qaeda's affiliates, Islamic militants now claim an American carried out a suicide attack against a regime military checkpoint. CNN's Mohammed Jamjoom reports.

As the fight against al Qaeda in Yemen flares into what looks dangerously close to all-out war, the U.S. is bracing itself for more possible terror attacks. CNN has learned exclusively that the threat against the U.S. Embassy in Sana'a, the country's capital, is far more serious than originally thought.

The Embassy announced on Wednesday it was shutting its doors to the public indefinitely. Now, two Yemeni National Security officials tell CNN the plot against the Embassy is bigger than it was believed to have been.

"The U.S. government is taking this threat far more seriously than they've taken other credible threats against them," explained one of the officials who requested anonymity as he was not authorized to speak with the media.

Of great concern to Americans in Yemen is the increase in fighting in Sana'a. According to numerous officials, on Friday al Qaeda linked militants attacked two government installations – first the Political Security Headquarters, where they engaged in a firefight with Yemeni troops, and then outside the Presidential Palace, where huge clashes with the military continued. At least 4 Yemeni troops and at least 3 militants died as a result, with the death toll expected to rise. Interior Ministry and Defense Ministry officials worry the situation is deteriorating dangerously.

Earlier in the week, al Qaeda in the Arabian Peninsula, or AQAP, targeted foreign installations in the capital – killing one French citizen who worked with the European Union Mission there.

The officials added the U.S. is taking this as a graver threat than the one encountered in 2012, when protesters breached the Embassy's heavily secured perimeter and stormed the building.

According to terrorism experts, there's more reason than ever for the U.S. to worry.

"The crown jewel of their organization, their master bomb maker, who keeps making these very sophisticated bombs that get on planes – there's no evidence he's dead," explained CNN National Security Analyst Peter Bergen.

Despite years of hunting him, Saudi born Ibrahim Al-Asiri, the man who tried to bring down a US plane with an underwear bomb in 2009 is thought to still be at large.

Of all al Qaeda affiliated networks worldwide, AQAP is considered the most dangerous threat to Americans.

Yemen's military now must contend with battling these terrorists not just in the capital – they're also continuing ground operations against AQAP in southern provinces such as Abyan and Shabwa. Those began three weeks ago and are ongoing.

"AQAP is a grave threat to both Yemeni and American security," explained US State Department Spokeswoman Jen Psaki at a briefing on Friday, "and the U.S. government welcomes the actions of Yemen's brave forces to counter this group."

The U.S. may be grateful but one high level Yemeni government official briefed on the counter terror operations says his country's allies – like the Americans and the Saudis – should be doing more.

"Yemen is asking all our allies and our friends for more support (for) the military in it's fight against AQAP," said the official, who also requested anonymity as he was unauthorized to speak to the media.

Added the official, "the fight against AQAP is a real struggle we are engaged in, it is not easy – we are even facing fuel shortages. These shortages don't just affect the whole country, they also affect the military in it's fight against AQAP. That means ground troops fighting AQAP in places like Shabwa & Abyan might literally run out of fuel and then might not be able to keep pursuing AQAP."

The official described the situation as dire.

When asked if Saudi Arabia or the US had provided support since ground troops went into these provinces, he said: "No, not in this battle".